1. Field of the Invention
This invention is directed to producing a fully dense ceramic laminate comprising a thermally conductive ceramic in layered contact with one or more layers of ceramic glass composite and the resulting unique product. Any or all of the layers may be metallized and as such can contain electrical circuit patterns, ground planes, etc. as required to build electronic packages for integrated circuit devices.
2. Description of the Previously Published Art
Some of the basic functions of electronic packages (or substrates) are to provide mechanical support for, provide electrical connections for, and dissipate heat generated by integrated circuit (IC) devices. Mechanical support is necessary to facilitate handling and prevent breakage of the IC during installation or operation of the device. Electrical connections (or wiring) on the packages provide power inputs, interconnects between devices, ground planes, etc. for the IC. Heat dissipation is required to avoid severe thermal excursions which could result in decreased reliability and device failure.
IC performance trends, which include increased function density, power density and speed, are placing more stringent requirements on the electronic package functions. These trends are resulting in the need for packages which have increased wiring density and improved thermal management capability. As a consequence of this an electronic package should consist of a dielectric support with better dielectric and thermal performance. Lower dielectric constants are desirable to decrease signal propagation delays in the conductor lines, which limit operating speeds. Lowering of the dielectric constant would also decrease electrical noise resulting from capacitance between neighboring conductor lines. This becomes more critical as the wiring density increases.
Thermal performance of the package is also an important consideration. High thermal conductivity is desired to avoid severe heating of the device, which can compromise both performance and reliability. Coupled with this is a desire to match the thermal expansion of the package with that of the IC, since some heating is unavoidable. Poor thermal expansion matching can result in substantial stresses and mechanical failure at the IC/package interface. The predominant IC material at present is silicon, which has a thermal expansion coefficient of approximately 3.times.10.sup.-6 /.degree. C. from room temperature to 300.degree. C.
Ceramics are often chosen for high reliability packaging because, in general, they provide the best compromise of the above-mentioned properties within affordable processing constraints. Ceramic packages often offer the capability of producing a hermetic package, which can provide superior protection of the IC from harsh environmental conditions and is often required for high reliability applications. They also have a higher Young's modulus than plastic packages, which provides more mechanical protection for the IC device.
The importance of dielectric constant and thermal conductivity for electronic packaging has resulted in two broad classes of ceramic packaging materials: (1) those possessing a relatively high thermal conductivity, but a relatively high dielectric constant and (2) those possessing low dielectric constants, but also a relatively low thermal conductivity. Choice of ceramic material for electronic packaging often depends in part on the relative importance of thermal conductivity and dielectric constant for the specific application.
Within the class of thermally conductive ceramic packaging materials, alumina (Al.sub.2 O.sub.3) is clearly the most commonly used. Other available choices include beryllia (BeO) and (more recently) aluminum nitride (AlN). Processing of these materials is characterized by relatively high sintering temperatures (approximately 1400.degree. C. to 2000.degree. C.). Hence, they are co-firable only with refractory metals (e.g., tungsten, molybdenum). Table 1 lists representative properties for these materials. Each has attributes which can make it the desired choice for specific applications. For instance, Al.sub.2 O.sub.3 is most often chosen because it provides reasonably high thermal conductivity at the lowest cost. BeO and AlN are more expensive at present, but provide significantly higher thermal conductivities which may be required for extreme high power applications. BeO offers the additional advantage of having a significantly lower dielectric constant than the other two; however, the toxicity of its dust has limited its development and use to a great degree. Besides high thermal conductivity, AlN offers a much better thermal expansion match to silicon than either BeO or Al.sub.2 O.sub.3.
The approaches to make the low dielectric constant ceramics have generally been to use silicate-based materials since these have some of the lowest dielectric constants for ceramic materials. Many of the silicate-based ceramics offer an additional advantage in that they sinter to near full density at relatively low temperatures (e.g., 800-1000.degree. C.). This makes them compatible with the sintering of more conductive metals such as copper, gold and silver. Use of these conductive metals results in a decrease in localized heating within the conductor lines and a decrease in voltage drops across these lines. This can be of significant importance for packages designed with a high density of fine wiring. One problem with silicate-based materials, however, is that they typically suffer from low thermal conductivity. To counteract this, composites which include a thermally conductive particulate material in a silicate matrix are being considered to provide a better balance between dielectric constant and thermal conductivity. The thermal conductivities of such composites reported in the literature are much less than that of alumina. This limits their use significantly.
Table 1
TABLE 1 ______________________________________ Thermally Conductive Ceramic Packaging Materials Representative Properties Thermal Thermal Expansion Conductivity Dielectric Coefficient Material (W/cm .degree.K.) Constant (.times.10.sup.6 /.degree.C.) ______________________________________ Al.sub.2 O.sub.3 0.25-0.38 10 6-7 BeO 1.50-2.50 6.7 6-7 AlN 0.70-2.50 9 4 ______________________________________
Examples of the ceramic/glass composite approach to electronic packaging can be found in the literature. For instance, researchers have described a glass-ceramic material with a dielectric constant of 7.5 which can be sintered in air at 900.degree. C. and as such is compatible with gold and silver-palladium metals. (See NEC Res. & Develop., No. 75, Oct. 1984, pp. 8-15.) They report a thermal conductivity of 0.042 W/cm.degree.K. and a thermal expansion coefficient of 42.times.10.sup.-7 /.degree. C. (from room temperature to 250.degree. C.). They indicate that the composite is a 55:45 ratio of alumina to lead borosilicate glass by weight. Combined with density information given, this translates to approximately a 46:54 alumina to glass ratio by volume.
European Patent Application 253 342, published Jan. 20, 1988, Nair, K.M., discloses thick film glass ceramic dielectric compositions comprising aluminoborosilicate glass and AlN.
U.S. Pat. No. 4,313,026 describes a multilayer circuit board with dielectric layers consisting of a borosilicate glass-alumina composite and an alumina sintered plate. They report that at least 50 wt. % of the glass can ensure a high density of the composite. At 40 wt. % glass, the reported composite density is less than 80%. The most preferable glass/ceramic composite was that consisting of 50 wt. % glass and 50 wt. % alumina. This, combined with density information given in the patent, translates to approximately 65 volume % glass and 35 volume % alumina. Properties of this preferred composition were a thermal conductivity of 0.04 W/cm.degree.K., a thermal expansion coefficient of 45.times.10.sup.-7 /.degree. C. and a dielectric constant.
Researchers at E. I. DuPont de Nemours & Co. describe the properties of a low temperature firing ceramic composite for electronic packaging applications, which is co-firable with gold, silver, and silver-palladium metals. (See Eustice, A. L. et al, Low Temperature Co-firable Ceramics: A New Approach for Electronic Packaging. Proceedings--Electronic Components Conference 36th Publication by IEEE, New York, NY, pp. 37-47 [1986].) Composition of the composite is unspecified, except to say that the matrix phase is a glass and the filler phase is a refractory material. Reported properties of this composite included a thermal expansion coefficient of 70.times.10.sup.-7 /.degree. C. and a dielectric constant of 8.0. The thermal conductivity of this composite is reported to be in the range of 15 to 25% of the value for 96% alumina. Assuming a value of 0.25 W/cm.degree.K. as the thermal conductivity of 96% alumina (reported in the literature), the reported percentages would correspond to 0.037 to 0.062 W/cm.degree.K. for this composite's thermal conductivity.
Homogeneous 3-component blends of ceramic particles, polymers, and mineral oil are well-known, including formation into sheets and removal of the oil by heating or by solvent extraction. See, e.g., U.S. Pat. No. 3,904,551.
The present invention represents a distinct improvement over the old art described above by providing a technique for making multilayer ceramic electronic packages comprised of a thermally conductive base, layers of low dielectric constant composite and conductive metallization. The low dielectric constant material can support layers of electrical circuits on the package and the thermally conductive base is effective in dissipating heat from the IC device.